RWestin
If I remember correctly, Westin uses a Bolt, with an attached nut, which goes behind the rocker panel. Three mounts, or braces, for steps and just two mounts, or braces, for the bar type.
Problem with the rocker mount design is simply the sheet metal itself is far too thin. That flexing you feel is, most likely, the rock panel itself giving way to the stresses involved.
Over time, many have actually pulled their step bar mounts completely through the rocker panel. Think a piece of tin that you keep flexing, until it breaks. Same thing is going on with the rocker panel , if you feel any flexing at all.
Reason AMP Research has introduced a plate template which covers the rocker mount location, for their mounts. This template gives added strength to the rocker panel metal.
This whole issue of step bars tearing away from the truck is why I have yet to install a set and am still researching the very best method of mounting.
Right now, I am leaning towards having my own extra-strength plates made, to cover over the factory mounting locations, then have them welded in place,. Am seriously considering a cab mount bracing bar, to go from the two cab mounts, up against the front and rear step braces.
Good luck on your install and let us know hos it turns out, as time goes by.
Dave
PS
Not trying to be a downer. Hoping to inform and prevent an issue, for you and any others who read this thread, years or so down the road.
Some of this I read here, on this very forum. Worried me enough, that I went to the Dee Zee factory outlet store, across the street from the DEE ZEE factory itself, here in town, and talked with their installers. They were adamant that I go with a bolt bar and nut AND a cab mounting system. Specifically told me to stay away from their Zerx type inserts. The Bolt and nut bar, behind the rocker, is stronger mounting. According to them. Add in the cab mount brace and better yet. Though none of that addresses the issue of too thin and weak rocker panel itself.
Reason I am thinking of adding something along the liens of AMP Research template too.